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What is the difference between sensation and
perception?
Sensation:
Perception:
The process of detecting a
stimulus, such as
• light waves (vision)
The process of integrating,
organizing and interpreting
sensations.
,
,
• sound waves (hearing)
• chemical molecules (smell
and taste
),
• heat or pressure (touch).
You might want to think of sensation and perception as
two ends of a continuum. There is no clear dividing line
between sensation and perception. Where sensation
ends and perception begins is difficult to determine.
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Signal Detection Theory
Signal detection theory states that detecting a stimulus [by
a person] is not an objective processes (page 178).
Detecting a stimulus is a subjective decision involving (1)
sensitivity to the stimulus in the presence of distractions
from other stimuli (noise) and (2) the criteria used to make
judgments from ambiguous information (also see
response bias, page 178).
When detecting light stimuli,
(1) The reality of the light is that it is on or off, and
(2) you must decide if it is on or off.
th
Image source: Psychological Science, 4 edition, (2013), Gazzaniga, Heatherton, and Halpern
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Signal Detection: Intensity of the Stimulus
Higher Intensity
Light presented
Light not
presented
Your response
Your response
yes, yes,
yes, yes,
yes, yes,
yes, yes
no, no
yes, yes
no, no,
no, no, no,
no, no, no
Lower Intensity
Your response
Your response
Light presented
yes, yes,
yes, yes,
yes, yes
no, no,
no, no
Light not
presented
yes, yes,
yes, yes
no, no, no,
no, no, no
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Signal Detection: Response bias
Liberal: You are more likely to say “yes”. Your standards
are more lax.
Your response
Light presented
yes, yes,
yes, yes,
yes, yes,
yes, yes
Your response
no, no
yes, yes,
no, no
yes, yes,
yes, yes,
yes, yes
There is a higher risk of false positives (false alarm) where you
will respond when there is no actual event (light)
Light not
presented
Conservative: You are more likely to say “no”. Your
standards are high.
Your response
Light presented
yes, yes,
Light not
presented
yes, yes
Your response
no, no, no,
no, no, no,
no, no
no, no, no,
no, no, no,
no, no
There is a higher risk of missing something that really is there
where you won’t respond to something that actually is there.
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Signal Detection: Response bias
Liberal: If people haven’t paid their taxes, was it an honest
mistake (Ming) and be forgiving?
Be forgiving
The person made Correct decision
an honest mistake
The person was
trying to cheat on
their taxes
false positive
Come down hard
on them
miss
Correct decision
There is a higher risk of false positives (false alarm) where you
will respond when there is no actual event (light)
Conservative: If people haven’t paid their taxes, did they
try to cheat the system (Boris) and need come down hard
on them?
Be forgiving
The person made Correct decision
an honest mistake
The person was
trying to cheat on
their taxes
false positive
Come down hard
on them
miss
Correct decision
There is a higher risk of missing something that really is there
where you won’t respond to something that actually is there.
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Signal Detection: Response bias
Liberal: You are more likely to accept new information,
regardless of whether it is accurate or not.
Believe it
New information is Correct decision
true
New information is
false
false positive
Dismiss it
miss
Correct decision
There is a higher risk of false positives (false alarm) where you
believe lots of things that are not true.
Conservative: You are less likely to accept new
information, regardless if it is true or not.
Believe it
New information is Correct decision
true
New information is
false
false positive
Dismiss it
miss
Correct decision
There is a higher risk of missing something that really true, but
don’t believe it.
Critical thinking can help you evaluate incoming
information. However, we are prone to use biased
thinking (see Chapter 1 and Psy 202).
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Signal Detection: Response bias
Signal Detection Theory: An observation that the response
to a stimulus depends both on
• Intensity of the stimulus
• on a person’s response bias.
Is the Light on?
Image source: Psychology (2009), Schacter et. al
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In the real world, we have to make judgments under
uncertainty such as school closures due to bad weather,
surgery for cancer, detection of enemy aircraft,
assessment of bomb threats, lie detection, etc.
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When distributing financial aid, there are some people who
may need it, and some who may not really need it.
Award financial
aid
Reject financial
aid
Sam needs
financial aid
Correct decision
miss
Chris doesn’t
need financial aid
false positive
Correct decision
Should you perform a medical procedure for breast
cancer?
Perform surgery
Not perform
surgery
you have breast
cancer*
Correct decision
miss
you have don’t
have breast
cancer*
false positive
Correct decision
* there is uncertainty in detection of cancer
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A decision at 5:00 am needs to be made: Should you
close the school?
Close school
Keep school open
Weather will get
worse
Correct decision
miss
Weather will get
better
false positive
Correct decision
Is the bomb threat real?
There is a bomb
There is NO bomb
Evacuate
Not evacuate
Correct decision
miss
false positive
Correct decision
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Should you attack an unknown aircraft?
Enemy aircraft
Friendly aircraft
Shoot aircraft
down
Hold your fire
Correct decision
miss
false positive
Correct decision
Is the person lying about stealing $100 when the lie
detector is “spiking”?
I think they are
lying
I think they are
telling the truth
They lied about
stealing
Correct decision
miss
They are telling
the truth about
stealing
false positive
Correct decision
What are other decisions that can be looked at with the
signal detection framework?
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How much product should I order?
I order a lot of
product
I order a small
amount of product
The product will
sell
Correct decision
miss
The product won’t
sell
false positive
Correct decision
As a police officer, do I shoot that person?
I shoot
I hold my fire
The person is a
criminal
Correct decision
miss
The person is not
a criminal
false positive
Correct decision
• is the person dangerous?
• Is the item a weapon?
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Sensory Processes
There are some basic concepts that psychologists use
when talking about the sensitivity of the senses (pages
177 - 178
• Sensory adaptation: A decrease in sensitivity to a
constant level of stimulation (page 178).
Sensory adaptation: The decline in sensitivity of the basic
senses to a constant stimulus. Therefore, a stronger
stimulus is required to activate the senses (another
definition).
Smell:
Hearing:
Touch
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Sensory Adaptation
How does sensory adaptation help us understand the
world around us?
Image source: unknown
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Sensory Adaptation
How does sensory adaptation help us understand the
world around us?
Image source: unknown
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Why is understanding sensory adaptation important
• in what cases would sensory adaptation be “bad”?
• in what cases would sensory adaptation be “good”?